Towards more precise Be and Cl data from measurements at the 10 level: Influence of sample preparation

The spreading application of accelerator mass spectrometry (AMS) to the geosciences will require measurement of increasing numbers of samples with low ¹⁰Be/⁹Be and ³⁶Cl/Cl isotopic ratios. To distinguish radionuclide concentrations in samples from corresponding processing blanks, samples must be prepared using stable isotope carriers with low intrinsic radionuclide concentrations. Measurements at different AMS facilities have shown that commercially available ⁹Be carriers rarely meet these requirements and use of ⁹Be carriers prepared from Be-containing minerals such as Be₂SiO₄ is advisable. For precise determinations at the low 10⁻¹⁴ level, samples need to produce the highest ⁹Be currents possible. Measurements performed at the two AMS facilities in France show that mixing BeO with Nb powder generally yields higher ⁹Be currents than mixing with Ag powder. Contamination of BeO by other elements such as Ti or Al will reduce the current by a simple dilution effect, thus limiting the statistical precision with which ¹⁰Be can be determined. In the case of ³⁶Cl analyses, repeated water-leaching of calcite samples sufficiently removes all atmospheric ³⁶Cl contamination, allowing determination of terrestrial cosmogenic in situ produced ³⁶Cl concentrations for surface exposure dating.     

Applications of a compact ionization chamber in AMS at energies below 1 MeV/amu

The increasing demand for measuring long-lived radionuclides with small AMS machines at energies below 1 MeV per nucleon raises the need for compact detectors which still have a decent energy resolution and allow for a clear identification of the incident particles. Based on a design by the AMS group at the ETH Zurich a compact gas ionization chamber was built and installed at the 3 MV tandem AMS facility VERA (Vienna Environmental Research Accelerator). The main challenge in AMS is the detection of rare isotope species in the presence of strong isotopic and isobaric interferences. The task of the ionization chamber is the suppression of the unwanted isobar by separating the ions via their different stopping powers. Measurements of ³⁶Cl at VERA showed an achieved suppression of the unwanted stable isobar ³⁶S of 3 × 10⁻⁴ and measurements of ¹⁰Be showed an achieved suppression of ¹⁰B of at least 3 × 10⁻⁶. Additional suppression of the isobaric ions can be achieved by a degrader foil technique applied to ¹⁰Be measurements by G.M. Raisbeck. In combination with the new ionization chamber the achieved suppression of ¹⁰B is at least 10⁻¹⁰. Measurements of blank samples at VERA show that the background for AMS with ¹⁰Be is below 2 × 10⁻¹⁵.     

First results from the nuclear astrophysics AMS program at the NSL using the MANTIS system in gas-filled mode

The Magnet for Astrophysical Nucleosynthesis studies Through Isobar Separation (MANTIS) system is the new Accelerator Mass Spectrometry (AMS) set-up created during recent upgrades of the Browne-Buechner spectrograph at the University of Notre Dame. Commissioning measurements performed on the separation of ⁵⁸Fe-⁵⁸Ni isobars at 114 MeV out of the FN tandem accelerator have shown clear separation, opening the door for a number of future measurements in nuclear astrophysics. The separation of mass-58 isobars has made this system the first in the world to utilise a Browne-Buechner spectrograph in gas-filled mode for AMS measurements with a special focus on nuclear astrophysics.     

Nucleogenic Cl, U and Pu in uranium ores

The nucleogenic isotopes ³⁶Cl, ²³⁶U and ²³⁹Pu are produced naturally in subsurface environments via neutron capture of thermal and epithermal neutrons. Concentrations are, however, very low and accelerator mass spectrometry (AMS) is required for quantitative measurements. A particular challenge is presented by the measurement of ²³⁶U/²³⁸U ratios down to the level of 10⁻¹³ that is expected from rocks with low uranium concentration. Here, we present the AMS methodology that has been developed at the ANU for measuring ²³⁶U/²³⁸U ratios at this level. The more established methodologies for ³⁶Cl and ²³⁹Pu measurements are also summarised. These capabilities are then used to characterize the ³⁶Cl, ²³⁶U and ²³⁹Pu concentrations in a range of uranium ores. A simple model of the neutron production and capture processes in subsurface environments has been developed and is presented. It is shown that nucleogenic ³⁶Cl, ²³⁶U and ²³⁹Pu can be used to determine both thermal and epithermal neutron fluxes in subsurface environments. Potential applications include uranium exploration and monitoring of the environmental impact of uranium mining.     

Natural and anthropogenic U in environmental samples

The interaction of thermal neutrons with ²³⁵U results in fission with a probability of ∼85% and in the formation of ²³⁶U (t_1/2 = 2.3 × 10⁷ yr) with a probability of ∼15%. While anthropogenic ²³⁶U is, therefore, present in spent nuclear fuel at levels of ²³⁶U/U up to 10⁻², the expected natural ratios in the pre-anthropogenic environment range from 10⁻¹⁴ to 10⁻¹⁰. At VERA, systematic investigations suggest a detection limit below ²³⁶U/U = 5 × 10⁻¹² for samples of 0.5 mg U, while chemistry blanks of ∼2 × 10⁷ atoms ²³⁶U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A).     

Selective detection of C by laser photodetachment mass spectrometry

In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect ¹³C⁻ ions in the presence of ¹²C⁻ ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of ¹³C⁻ and ¹²C⁻ ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70 cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach ¹³C⁻ ions. The number of detected ¹³C atoms was 13 times larger than the number of detected ¹²C atoms. The population of the excited, weakly bound ²D excited state of the C⁻ ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the ⁴S ground state C⁻ ion. Different applications of the LPMS method will be discussed in the paper.     

Accelerator mass spectrometry programme and related developments at the BARC-TIFR Pelletron accelerator

The accelerator mass spectrometry programme and the related developments based on the BARC-TIFR Pelletron accelerator is described. A segmented gas detector and beam chopper control electronics have been developed for conducting ³⁶Cl measurements. Distinct energy loss characteristics of the gas detector and beam chopper performance are presented. Also presented are the initial results for the measurement of ³⁶Cl in standard and samples of unknown concentration.     

Method based on isotope ratio mass spectrometry for evaluation of carbon activation in the reactor graphite

The general idea of this work is to introduce an evaluation method to restore the irradiation parameters of graphite or other carbonaceous materials using experimental and modelling results of ¹³C generation in the irradiated material. The method is based on coupling of stable isotope ratio mass spectrometry and computer modelling of the reactor core to evaluate the realistic characteristics of the reactor core such as the neutron fluence in any position of the reactor graphite stack or other graphite constructions.The generation of carbon isotopes ¹³C and ¹⁴C in the irradiated graphite of the RBMK-1500 reactor has been estimated by modelling of the reactor core with computer codes MCNPX and CINDER90. Good agreement of simulated and measured Δ¹³C/¹²C values in graphite of the central part of the reactor core indicates that the neutron flux (1.40 × 10¹⁴ n/cm² s) is modelled accurately in the graphite sleeve of the fuel channel. The simulated activity of ¹⁴C is compared with the one measured by the β spectrometry technique. Results indicate that production of ¹⁴C from ¹⁴N in the RBMK-1500 reactor is considerable and has to be taken into account in order to make proper evaluation of ¹⁴C activity. Measured ¹⁴C specific activity values correspond to 15 ± 4 ppm impurity of ¹⁴N in graphite samples from the RBMK-1500 reactor core.     

Electron spin resonance dating of human teeth from Toca da Santa shelter of São Raimundo Nonato, Piauí, Brazil

Results of the dating of fossil human teeth excavated from a shelter in the surroundings areas of the Serra da Capivara National Park, São Raimundo Nonato, Piauí, Brazil are presented. This shelter was partially excavated to search for more data that could improve the archaeological context of the Garrincho’s limestone hill sites, where the Toca do Gordo do Garrincho shelter provided two human teeth dated by conventional C-14 in (12,170 ± 40) yBP (years before present) and calibrated age (2 Sigma, 95% probability) 15,245-14,690 yBP (Beta 136204) [E. Peyre, C. Guérin, N. Guidon, I. Coppens, CR Acad. Sci. Paris, Sciences de la terre et des planètes/ Earth & Planetary Sciences 327 (1998) 335, [1]].This region is challenging the classical theories about the peopling of America. In agreement with such theories the first human groups would have arrived in America, through Behring, only about 20,000 years ago. The site Toca do Boqueirão da Pedra Furada is a rock-shelter situated at the cliff that is the frontier between the Pré-Cambrian São Francisco plain and the Devonian-Permian highlands of the Maranhão-Piauí basin. It was excavated from 1978 till 1988 and presented a very consistent stratigraphy, with thousand of lithic implements and hearths [F. Parenti, Le gisement quaternaire de la Pedra Furada (Piaui, Brésil), Stratigraphie, chronologie, évolution culturelle, Editions Recherches sur les civilisations, Paris, 2002, [2]; F. Parenti, Le Gisement Quaternaire de la Toca do Boqueirão da Pedra Furada (Piauí, Brésil) dans le Contexte de la Préhistoire Américaine Fouilles, Stratigraphie, Chronologie, Évolution Culturelle, Ph.D. diss, Ècole des Hautes Ètudes en Sciences Sociales, Paris, 1993, 411p, [3]; F. Parenti, M. Fontugne, N. Guidon, C. Guérin, M. Faure, Chronostratigraphie des gisements archéologiques et paléontologiques de Sao Raimundo Nonato (Piaui, Brésil): contribution à la connaissance du peuplement pléistocène de l’Amérique, Supplément de la Revue d’Archéométrie, 1999, p. 327, [4]].Seventy C-14 dates were obtained going from 59,000 yBP to 5000 yBP at the actual soil [G.M. Santos, M.I., Bird, F. Parenti, L.K. Fifield, N. Guidon, P.A. Hausladen, Quaternary Sci. Rev. 22 (2003) 2303, [5]]. Beneath the date of 59,000 yBP it was found 1.5 m of archaeological layers, till the rock base. The cobbles that composed the structure of the hearths placed on top of the rock base were dated by thermoluminescence and the date of them is 100,000 yBP [H. Valladas, N. Mercier, M. Michab, J.L. Joron, J.L. Reiss, N. Guidon, N., Quaternary Sci. Rev. 22 (2003) 1257, [6]; H. Valladas, N. Mercier, M. Michab, J.L. Joron, J.L. Reyss, N. Guidon, Datações por Termoluminescência de Seixos de Quartzo Queimados da Toca do Boqueirão da Pedra Furada (Piauí, Nordeste do Brasil), Fumdhamentos III, 1 (2003) 35 (in Portuguese), [7]]. Thus chronological information is of paramount importance to contextualize and to understand the many questions regarding the human occupation of these sites [N. Guidon, A.M. Pessis, G. Martin, O Povoamento Pré-Histórico do Nordeste do Brasil, Proposta da: Fundação Museu do Homem Americano, e do Núcleo de Estudos Arqueológicos, da UFPE, CLIO ARQ, Recife 1 (6) (1990) 123, [8]]. To obtain more information Toca da Santa shelter, the nearest site to Toca do Gordo do Garrincho shelter, was also excavated and three incomplete skeletons with teeth were found at 80 cm depth. The teeth were cleaned and acid etched and enamel completely removed from dentine and crushed in particles smaller than 0.5 mm. Enamel was irradiated with a Co-60 source and measured with an ESR spectrometer (X-band) to obtain the signal versus dose curve. The archeological doses obtained by the fitting were (5.94 ± 0.07) Gy and (5.97 ± 0.08) Gy. Using the ROSY program an age of 5,700 ± 200 yBP was found for both teeth.     

Investigation of proton induced reactions on niobium at low and medium energies

Niobium is a metal with important technological applications: use as alloying element to increase strength of super alloys, as thin layer for tribological applications, as superconductive material, in high temperature engineering systems, etc. In the frame of a systematic study of activation cross-sections of charged particle induced reactions on structural materials proton induced excitation functions on Nb targets were determined with the aim of applications in accelerator and reactor technology and for thin layer activation (TLA). The charged particle activation cross-sections on this element are also important for yield calculation of medical isotope production (^88,89Zr, ^86,87,88Y) and for dose estimation in PET targetry. As niobium is a monoisotopic element it is an ideal target material to test nuclear reaction theories. We present here the experimental excitation functions of ⁹³Nb(p,x)^90,93mMo, ^92m,91m,90Nb, ^88,89Zr and ⁸⁸Y in the energy range 0-37 MeV.The results were compared with the theoretical cross-sections calculated by means of the code ALICE-IPPE, EMPIRE-3, TALYS and with the literature data. The theory reproduces the shape of the measured results well and magnitude is also acceptable.Thick target yields calculated from our fitted cross-section give reliable estimations for production of medically relevant radioisotopes and for dose estimation in accelerator technology.     

Caesium isothermal migration behaviour in sintered titanium nitride: New data and comparison with previous results on iodine and xenon

Titanium nitride has been proposed as a fission product barrier in fuel structures for gas cooled fast reactor (GFR) systems. The thermal migration of Cs was studied by implanting 800 keV ¹³³Cs⁺⁺ ions into sintered samples of TiN at an ion fluence of 5 × 10¹⁵ cm⁻². Thermal treatments at temperatures ranging from 1500 to 1650 °C were performed under a secondary vacuum. Concentration profiles were determined by 2.5 MeV ⁴He⁺ elastic backscattering. The results reveal that the global mobility of caesium in the host matrix is low compared to xenon and iodine implanted in the same conditions. Nevertheless, the evolution of caesium depth profile during thermal treatment presents similarities with that of xenon. Both species are homogeneously transported towards the surface and the transport rate increases with the temperature. In comparison, iodine exhibits singular migration behaviour. Several assumptions are proposed to explain the better retention of caesium in comparison with both other species. The potential role played by the oxidation is underlined since even a slight modification of the surface stoichiometry may modify species mobility. More generally, the apparition of square-like shapes on the surface of the samples after implantations and thermal treatments is discussed.     

Activation cross sections of proton induced nuclear reactions on ytterbium up to 70 MeV

Cross sections of proton induced nuclear reactions on ytterbium were measured up to 70 MeV by using the standard stacked foil irradiation technique and high-resolution gamma-ray spectroscopy. Experimental cross sections and derived integral yields are reported for the first time for the ^natYb(p,xn)^{173,172mg,171mg,170,167}Lu, ^natYb(p,x)^{175cum,166cum}Yb and ^natYb(p,x)^{173ind,172ind,168,167cum,165cum}Tm reactions. No earlier experimental cross section data were found in the literature. The experimental data were compared to and analyzed with the results of the theoretical model code ALICE-IPPE. Production routes of medical radioisotope ¹⁶⁷Tm are discussed.     

Storing Matter: A new quantitative and sensitive analytical technique based on sputtering and collection of sample material

The Storing Matter technique, which is a new analytical technique for both organic and inorganic materials, consists in decoupling the sputtering of the specimen from the subsequent analysis step. The surface of the specimen to be analysed is sputtered by means of an ion beam. The particles emitted under the impact of these primary ions are deposited at a sub-monolayer level on a dedicated collector under UHV conditions. It is only in a second step that the deposited matter is analysed in analytical instruments (mainly dynamic and static SIMS). Depositing the matter sputtered from different samples or from different layers of a sample on a same collector makes Storing Matter a powerful tool to circumvent the well-known matrix effect in SIMS. Moreover, enhanced secondary ion emission can be obtained in the different SIMS analysis modes as the collector surface and thus the matrix is chosen with respect to the elements to be analysed and the analysis mode (M⁺, M⁻, cationisation for organic information, etc.). In order to allow for the different steps of the Storing Matter technique to be performed under optimized conditions, a dedicated prototype instrument has been developed at SAM. This paper presents the Storing Matter technique and the dedicated prototype instrument with a special focus on the sputter-deposition process used in this new technique.     

Magnetic texture determination by conversion electron Mössbauer spectroscopy with circularly polarized beam

A new construction of a CEMS detector is presented. The detector is dedicated to resonant polarimetric studies of samples in an external magnetic field. The construction permits investigations at different angles of radiation with respect to the sample surface. A possibility of magnetic texture measurements using circularly polarized radiation is demonstrated. An average square of the cosine 〈(γm)²〉 and an average cosine 〈γm〉 were determined and compared with results of independent measurements of the magnetic field inhomogeneity by a Hall probe. CEMS measurements performed on an isotopically enriched sample show a substantial influence of thickness effects on the measured spectra. An appropriate correction for a saturation effect was proposed and tested.     

Study of proton induced reactions on niobium targets up to 70 MeV

Niobium is a metal with important technological applications: use as alloying element to increase strength of super alloys, as thin layer for tribological applications, as superconductive material, in high temperature engineering systems, etc. In the frame of a systematic study of activation cross-sections of charged particle induced reactions on structural materials proton induced excitation functions on Nb targets were determined with the aim of applications in accelerator and reactor technology and for thin layer activation (TLA). The charged particle activation cross-sections on this element are also important for yield calculation of medical isotope production (^88,89Zr,^86,87,88Y) and for dose estimation in PET targetry. As Niobium is a monoisotopic element it is an ideal target material to test nuclear reaction theories. We present here the integral excitation functions of ⁹³Nb(p,x)^90,93mMo, ^92m,91m,90Nb, ^86,88,89Zr, ^86,87mg,88Y and ⁸⁵Sr in the energy range 30-70 MeV, some measured for the first time at this energy range.The results were compared with the theoretical cross-sections calculated by means of the code ALICE-IPPE and with the literature data. The calculations have been carried out without any parameter adjustment. The theory reproduces the shape of the measured results well and magnitude is also acceptable.Thick target yields calculated from our fitted cross-section give reliable estimations for production of medically relevant radioisotopes and for dose estimation in accelerator technology.     

Investigation of the Zn(p, 2p)Cu nuclear reaction: New measurements up to 40 MeV and compilation up to 100 MeV

The excitation function was measured for the ⁶⁸Zn(p, 2p)⁶⁷Cu nuclear reaction from its threshold energy up to 40 MeV. Nine pieces of highly enriched ⁶⁸Zn (>98%) metal foils were irradiated to obtain reliable cross-sections using the usual stacked-foil technique. All foils were subjected to high efficiency radiochemical separation before the activity measurements. A critical compilation of the available experimental cross-section results was also performed. Thick target yields of ⁶⁷Cu and the longer-lived copper radio-contaminants (⁶¹Cu and ⁶⁴Cu) were calculated using the reliable literature results up to 100 MeV. Additionally, EOB (End Of Bombardment) contamination levels as a function of bombarding energy and irradiation time were deduced.     

Optical degradation of silicone in ZnO/silicone white paint irradiated by <200 keV protons

By compared with ZnO powders and silicone resin, the optical degradation of the ZnO/silicone white paint caused by <200 keV protons was investigated, and the damage mechanism was revealed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrum analysis (FTIR). Experimental results show that the optical degradation mainly occurs in the visible region and the absorption band shifts towards longer wavelengths with increasing proton fluence. Under the proton irradiation, the resistance to optical degradation of the ZnO/silicone white paint is less than the ZnO powders and better than the silicone resin. Such a change can be related to the damage of the silicone. The damage of the silicone binder enhances the degradation of ZnO/silicone white paint, whereas the existence of ZnO pigment relieves the degradation of the silicone binder. The major damage product changes from the cyclotrisiloxane in the silicone resin to the net Si-CO structure in the silicone binder of the ZnO/silicone white paint.     

The new sample preparation line for radiocarbon measurements on atmospheric aerosol at LABEC

Radiocarbon measurements on the carbonaceous aerosol fractions have been demonstrated as an effective tool for aerosol source apportionment. For these measurements, a new sample preparation facility was installed at the INFN-LABEC laboratory of Florence (Italy). The line was designed to allow the preparation of samples from different carbonaceous fractions: the combustion of the aerosol samples can be performed in helium or oxygen flows, according to thermal sequences. The evolved CO₂ is cryogenically trapped and reduced to graphite, which is the target material for following Accelerator Mass Spectrometry (AMS) ¹⁴C measurements. This preparation line is described in detail in the paper. As a first step, the line was tested by means of AMS measurements performed on standards to check the reproducibility and the accuracy of the system; moreover, preliminary measurements on the total carbon fraction in aerosol samples were made. Results of these measurements are also reported.     

A study on the degradation of GaAs/Ge solar cells irradiated by <200 keV protons

Damage effects in GaAs/Ge solar cells irradiated by <200 keV protons were studied by measuring their electrical properties and spectral response together with SRIM simulations. Proton energies of 40, 70 and 170 keV were chosen. Experimental results show that the short circuit current, open circuit voltage and maximum power decrease with increasing proton fluence. The degradation of the open circuit voltage is highest for 70 keV irradiation and lowest for 40 keV irradiation. The degradation of short circuit current decreases with increasing proton energy. According to SRIM simulations and spectral response analysis, the above changes in electrical properties are mainly related to damage in different regions of the solar cells.     

Protactinium-231: A new radionuclide for AMS

For the first time, an AMS system was used to determine concentrations of the actinide Protactinium-231. ²³¹Pa has widespread applications in the earth sciences. It can be used for U-series disequilibrium dating, and ²³¹Pa is an important tracer in Paleoceanography. The Pa isotopes were measured with the compact ETH/PSI-AMS facility TANDY operating at about 300 kV. The linearity of the AMS is documented with two standard-dilution experiments and first measurements of natural samples from very different climate archives are presented. Our results show that it is possible to determine ²³¹Pa amounts in the lower femtogram (1 fg = 10⁻¹⁵ g) range with the TANDY. In natural samples, ²³¹Pa concentrations between 1 and 4 pg/g (1 pg = 10⁻¹² g) were measured with an average total error of 3% (one sigma). The average over all reproducibility of these first measurements was better than 2% with clear potential for improvement.